Constructing copper-zinc interface for selective hydrogenation of dimethyl oxalate

Journal of Catalysis 383 (2020) 254–263

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Constructing copper-zinc interface for selective hydrogenation of dimethyl oxalate Xuepeng Wang a,1, Meng Chen b,1, Xingkun Chen a,⇑, Ronghe Lin a,⇑, Hejun Zhu b, Chuanqi Huang a, Wenshao Yang a, Yuan Tan a, Saisai Wang a, Zhongnan Du a, Yunjie Ding a,b,c,⇑ a b c

Hangzhou Institute of Advanced Studies, Zhejiang Normal University, 1108 Gengwen Road, Hangzhou 311231, PR China Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China The State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China

a r t i c l e

i n f o

Article history: Received 27 December 2019 Revised 17 January 2020 Accepted 20 January 2020

Keywords: Hydrogenation Dimethyl oxalate Ethylene glycol Copper-zinc interface Structure-performance relationship

a b s t r a c t The development of robust catalysts for selective hydrogenation of dimethyl oxalate (DMO) is key for the expansion of ethylene glycol (EG) production from C1 and renewable feedstocks. Copper-based catalysts promoted with Cr are industrially used, but it is desirable to develop more environmentally-benign alternatives. Silica-supported copper-zinc bimetallic catalysts featuring intimate metal contact are designed and used to establish the structure-performance relationships through combining in-depth characterizations with steady-state catalytic testing. Our study highlights: i) that the surface Cu0/Cu+ ratio is a reliable catalytic descriptor that determines the activity and selectivity, and ii) the importance of copper-zinc interface in stabilizing copper nanoparticles both under reduction and hydrogenation conditions. Controlled zinc doping allows fine tuning of these properties for the targeted reaction. Excellent performance has been achieved on the best-performing catalyst with >99.6% DMO conversion, >96.0% EG selectivity, and unprecedented stability of 800 h without catalyst deactivation, which represents a significant advance in the selective hydrogenations. Ó 2020 Elsevier Inc. All rights reserved.

1. Introduction Ethylene glycol (EG) is a bulk commodity serving as a key building block in the manufacture of polymers, agrochemicals and pharmaceuticals. The annual world production reached 37.5 Mton in 2016, with a increasing rate of 4.5% per annual foreseen for the next years [1,2]. To date, its manufacture still heavily relies on the ethylene oxide hydration technology [3,4]. However, this is not practical in China considering the shortage of crude oil resources, and hunting alternative synthesis routes with cheap feedstock, such as CO coupling reaction to dimethyl oxalate (DMO) and its subsequent hydrogenation, is more attractive to fulfill the increasing demand for EG [5]. Additionally, the new route is advantageous considering that it can shift the feedstock from oil to coal or even biomass. In this context, encouraging results have been achieved as exemplified by the industrial demonstration of EG production (2.18 Mton/annual) via the coal-based route in 2017 in China. Still, numerous obstacles should be tackled to ⇑ Corresponding authors at: Hangzhou Institute of Advanced Studies, Zhejiang Normal University, 1108 Gengwen Road, Hangzhou 311231, PR China (Y. Ding). E-mail addresses: [email protected] (X. Chen), [email protected] (R. Lin), [email protected] (Y. Ding). 1 Equal contribution. https://doi.org/10.1016/j.jcat.2020.01.018 0021-9517/Ó 2020 Elsevier Inc. All rights reserved.

expand the wide industrial applications. Among these, the design of highly active, selective, and stable DMO hydrogenation catalysts is key [6–8]. Different catalytic systems have been studied for selective DMO hydrogenation, with attentions mainly centered on Ag- and Cubased catalysts. The former are mainly employed for the hydrogenation to methyl glycolate (MG), a primary hydrogenation product, as a result of the relatively mild hydrogenation performance [9–12]. Copper-based catalysts possess excellent hydrogenation activity for C=O bonds and are widely applied in the hydrogenation of various substrates including esters, carboxylic acids, furfural, and CO2 [13–17]. Owing to the high catalytic activity and low cost, Cu/SiO2 catalyst received much attention in the DMO hydrogenation to EG, a secondary hydrogenation product. In this context, establishing the structure-performance relationship of copper-based catalysts is key for rational catalyst design, nonetheless, systematic studies are scarcely conducted. Previous works prove that, in general, good copper dispersion and an appropriate Cu+/Cu0 ratio are crucial to achieve high activity and selectivity [1–5,18–24]. Besides, Gong et al. studied the monometallic Cu/SiO2 system and proposed the synergistic effect between Cu0 and Cu+ species where the metallic copper species are the active sites to activate molecular H2, while the DMO molecules can be

X. Wang et al. / Journal of Catalysis 383 (2020) 254–263

polarized by the Cu+ species [25,26]. Further works centering on the search for reliable catalytic descriptors for activity, selectivity, and particularly stability, are indispensable for optimization of the hydrogenation performance. Albeit showing high hydrogenation activity in DMO hydrogenation, the un-promoted copper-based catalysts unfortunately suffer significant deactivation. The loss of active metallic copper species is the main cause for the activity loss. Under elevated reaction temperatures, the copper particles tend to aggregate due to the weak interaction between copper species and the support, as well as the lower Hüttig temperature of copper, altogether leading to catalyst sintering deactivation [5,27–29]. In addition, valence transition of Cu species can occur as a result of the redox reaction between surface Cu species and the reactants during the hydrogenation and impacts the stability. Owing to the high durability, copper-chromium is the preferred industrial catalyst applied in early stage of DMO hydrogenation to EG. Nonetheless, the decreased activity upon Cr addition plus its toxicity stimulate a continued interest in hunting for more environmentally-benign catalysts [30,31]. To overcome these drawbacks, several strategies have been developed to improve the lifespan of Cu-based catalysts, mainly by enhancing the metal-support interactions or by introducing a promoter to stabilize the Cu particles. For instance, high DMO conversion and EG selectivity have been achieved by doping the Cu/SiO2 catalysts with Zn [20,32], B [33,34], Ag [35], Mg [36], Ni [37,38], Co [14], La [39] and Ce [40] with excellent stability even up to 300 h. These exciting results further demonstrate the great industrial potential of the copper-based systems for DMO hydrogenation albeit further improvement of the stability and evaluation of the catalysts in technical form are still needed. The DMO hydrogenation constitutes several consecutive hydrogenation steps (Scheme 1), therefore fine tuning of these properties by tailoring the compositions of the multi-component systems is particularly critical to achieve a balanced hydrogenation ability for both high activity and EG selectivity. Herein, a series of silicasupported copper-zinc catalysts with intimate contact between the metals, prepared through ammonia evaporation followed by impregnation, was targeted for establishing the structureperformance relationships of the bimetallic system. The selection of the promoter was based on two considerations: i) zinc has been successfully applied as a dopant in copper-based catalysts with improved catalytic performance in diverse reactions (eg., water gas shift [41], hydrogenation of carbon dioxide to methanol [42,43], methanol steam reforming [17], etc); and ii) previous works reported the strong interactions between Cu and Zn that may form a stable interface [44–47], thereby further assessing the impacts on the stability of copper nanoparticles and hydrogenation performance is valuable but not done yet. We demonstrated that the electronic and structural properties of the catalysts can be well tailored by carefully tuning the composition of the bimetallic catalysts following the developed catalyst synthesis protocol, unraveled the catalytic descriptors for the activity and selectivity, and revealed the importance of Cu-Zn interface for stabilizing copper nanoparticles under both reduction and hydrogenation conditions. Excellent catalytic performance in the vapor-phase chemoselective hydrogenation of DMO to EG can be achieved on the optimized catalyst, affording unprecedented stability up to 800 h without apparent catalyst deactivation.

2. Experimental section 2.1. Catalyst preparation The mono-metallic M/SiO2 catalyst (M = Cu, Zn, nominal metal content 20 wt%) was prepared using the ammonia evaporation

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(AE) method as previously described [31,32]. In a typical synthesis, 12.6 cm3 ammonia solution (25–28 wt% NH4OH, Sinopharm company) was mixed with 70 cm3 0.3 M copper nitrate solution ((Cu (NO3)2
3H2O, >99.0 wt%, Sinopharm company) and stirred for 5 min. The pH of the complex was about 11. Then 11.52 g fumed silica (SiO2, AEROSIL 300) was added to the copper ammonia complex solution and the mixture was stirred at 35 °C for another 4 h. After this, the suspension was heated to 90 °C to evaporate the ammonia until the pH decreased to 6–7. The solid was filtered, washed with 500 cm3 of deionized water, dried at 120 °C for 10 h, and calcined in air at 450 °C (4 h, ramping rate 1°C /min). The Zn/SiO2 catalyst was prepared following the above procedures by replacing copper nitrate with zinc nitrate hexahydrate ((Zn (NO3)2
6H2O, 99 wt%, Aladdin industrial corporation). The bimetallic x-Zn-Cu/SiO2 catalysts with intimate contact of the metals, where  represents the Zn content in wt.%, were prepared by an incipient impregnation method. Typically, a calculated amount of an aqueous solution of zinc nitrate was added to the above prepared Cu/SiO2 catalyst, and the mixture was aged at room temperature overnight, dried at 120 °C for 10 h, and calcined in air at 450 °C (4 h, ramping rate 1°C /min). An additional 2.0-Zn-Cu/SiO2-ref catalyst with poorer Cu-Zn contact was also prepared for reference purpose, following the above-described method without aging treatment. All the catalysts were well ground and sieved into 20– 40 mesh pellets before catalytic test. 2.2. Catalyst characterization The contents of Cu and Zn in the catalysts were determined by an inductively coupled plasma optical emission spectrometer (ICPOES, PerkinElmer, and Optima 7300 DV). Nitrogen sorption was measured at 77 K on a Quanta chrome NT3LX-2 instrument after degassing the samples at 400 °C for 3 h. The specific surface area and the pore size distribution were calculated by the BrunauerEmmett-Teller (BET) and the DFT methods, respectively. Fouriertransform infrared (FT-IR) spectroscopy analysis was performed on a Nicolet iS50 spectrometer using the KBr pellet method with the mass ratio of KBr to sample of 20. The spectra were collected in the range of 400–4000 cm1 with 32 scans and a resolution of 2 cm1 at room temperature. X-ray diffraction (XRD) analysis was conducted on an X’Pert 3, PANalytical X-ray diffractometer using Cu Ka radiation in a scanning angle (2h) range of 10-90° at a speed of 0.2° min1. The tube voltage and the current were 40 kV and 40 mA, respectively. For in situ XRD, the catalyst precursor was placed in a reaction cell manufactured by Anton Parr (XRK 900). H2 was introduced at a flow rate of 30 cm3 min1, and the temperature ramping programs were performed from room temperature to 25, 100, 140, 160, 200, 230, 250, 300, 350, 400, 500, 600 °C with a heating rate of 5 °C min1. The XRD patterns were collected after sample temperature reached the preset value for 10 min. The crystallite size of Cu nanoparticles was calculated by Sherrer equation using the Cu (1 1 1) facet at 43.2° 2h following Eq. (1):

D¼

kk Bcosh

ð1Þ

where, k is the Scherrer constant (0.89), k is the wavelength of X-ray (0.154056 nm), B is the Full Wave at full width at half maximum of diffraction peak, and h is the diffraction angle. The specific surface area of the copper active site was determined by N2O pulse titration on a Micromeritics Autochem II 2920 equipped with a thermal conductivity detector. Briefly, the sample (100 mg) was treated in a high purity He gas stream (30 cm3 min1) at 200 °C for 1 h and then reduced in a 10% H2/ Ar stream at 300 °C for 3 h, and the hydrogen consumed was recorded as S1. Next, the sample was cooled to 50 °C under Ar,

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Scheme 1. Consecutive reactions involved in the hydrogenation of DMO to EG.

and oxidized with a 10% N2O/N2 mixture (50 cm3 min1) for 2 h to ensure complete oxidation of metallic copper. Finally, the sample was heated to 300 °C under Ar flow, and reduced by 10% H2/Ar pulse at the same temperature. The hydrogen consumed in this session was recorded as S2. The surface area of metallic copper was calculated following Eq. (2):

SCu0 ¼

2  S2  N A 1353  S2 ¼ N  S1  MCu  W S1  W

ð2Þ

where, N is the number of atoms per m2 of active Cu surface (1.46  1019), NA is Avogadro constant, MCu = relative atomic mass (63.456 g/mol), and W is mass percentage of Cu in wt.% [16,48]. Hydrogen temperature-programmed reduction (H2-TPR) and temperature-programmed desorption (H2-TPD) were studied on a Micromeritics Autochem II 2920 chemisorber equipped with a thermal conductivity detector. For H2-TPR, the sample (100 mg) was pretreated in a quartz U-tube reactor with a He gas stream (30 cm3/min) at 200 °C for 1 h. After cooling to room temperature, a 10% H2/Ar flow (50 cm3/min) was introduced and the sample was heated to 800 °C (10 °C /min). For H2-TPD, the sample (100 mg) was first reduced in 10% H2/Ar flow (50 cm3/min) at 230 °C for 3 h, and then cooled to 80 °C and kept for another 1 h in the reductive atmosphere to allow complete hydrogen adsorption. Finally, the sample was heated to 800 °C (10 °C /min) in He. The surface topography images of the sample were obtained by Tecnai G2 F30 transmission electron microscope (TEM). The sample powder was dispersed in ethanol by ultrasonication and then the specimen was obtained by dropping a droplet suspension on a carbon film supported on a nickel grid for TEM analysis. Elemental mapping analysis was performed on JSM-7800F field emission scanning electron microscope and high-angle-annular-dark-field scanning transmission electron microscopy (HAADF-STEM) equipped with energy dispersive spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS) and the X-ray Excited Auger electron spectroscopy (XAES) analysis of the catalysts were carried out on a Thermo ESCALAB 250Xi spectrometer using a 15 kV Al Ka X-ray source as a radiation source. The binding energy was calibrated using the C 1s peak (284.6 eV) as the reference.

7890B chromatograph equipped with an HP-FFAP capillary column (30 m  0.32 mm  0.25 lm) and a flame ionization detector (FID) using n-butanol as the internal standard. 3. Results and discussion 3.1. Compositional and structural properties of the catalysts The metal contents of mono- and bimetallic catalysts measured by ICP-AES showed that the copper loadings are all similar (ca. 22 wt%, Table 1), close to the nominal value. This suggests that most of the copper species precipitated from the Cu(NO)3
H2O complex into copper phyllosilicate and Cu(OH)2 during the synthesis. In addition, the Zn contents of the catalysts were nearly equal to the preset values. The phase compositions of the calcined and reduced catalysts were then studied by ex situ XRD. No obvious diffraction peaks for Zn species is observed in all the cases except the broad peak at 22° 2h associated with the silica carrier. For the calcined catalysts, broad and weak peaks at 30.8° and 35.2° 2h are observed, corresponding to the diffractions of copper phyllosilicate phase [Cu2SiO5(OH)2] (Figure S1) [49]. This result is further corroborated by FTIR analysis, as the dO-H band at 670 cm1 is indicative of the broadly similar aboudance of copper phyllosilicate (Figure S2). After reduction in hydrogen, the characteristic diffractions of Cu0 at 43.3°, 50.4°, and 74.1° 2h, corresponding to the respective (1 1 1), (2 0 0), and (2 2 0) planes, are observed in all the catalysts. Additionally, theses peaks become broader as the Zn content increases. The textural properties of the calcined catalysts were analyzed by N2 sorption (Table 1 and Figure S3). The N2 adsorption–desorption isotherms of the catalysts exhibited Langmuir type IV isotherms with H1-type hysteresis loops which are similar to that of the SiO2 support [50,51]. Besides, the shape of the hysteresis loops and the pore size distribution did not change much with the increasing Zn content. All the Cu-based catalysts synthesized by the AE method showed much higher surface area and larger pore volume than the SiO2 support, probably due to the formation of copper phyllosilicate. In contrast, the Zn/SiO2 shows significantly reduced surface area and total pore volume likely because of the blockage of channels or pores by zinc.

2.3. Catalytic test 3.2. Particle size and copper dispersion The catalytic vapor-phase hydrogenation of DMO was performed in a micro-reaction system over a stainless steel fixedbed reactor (inner diameter, 10.0 mm; length, 660 mm), equipped with a thermocouple and a mass flow controller for the control of reaction temperature and hydrogen flow rate. Typically, 1.0 g of the catalyst was placed in the center of the reactor, and reduced in situ with H2 (30 cm3/min, 230 °C, 0.1 MPa, 3 h). After cooling to the reaction temperature, the system pressure was increased to 1.5 MPa with a back-pressure regulator. Then, a DMO solution (20 wt% DMO in methanol, 99%, Analytic Reagent) was admitted using a plunger pump (NP-KX-210, Nihon seimitsu kagaku co., ltd). The weight liquid hourly space velocity (WLHSV) based on DMO was in the range of 0–3.5 h1. The reaction products were collected by a cold trap and analyzed offline with an Agilent

The manipulation of copper particle size by zinc was studied by combining in situ XRD of the calcined catalysts in flowing hydrogen (Fig. 1) and TEM of the reduced ones (Fig. 2). With elevated temperatures, the characteristic peak of Cu0 at 43.3° 2h appeared with simultaneously disappeared Cu2SiO5(OH)2 peaks for both Cu/SiO2 and 2.0-Zn-Cu/SiO2 catalysts when the reduction temperature reached 160 °C. The copper diffraction peak became sharper with increasing reduction temperature because of the accumulation and coagulation of the metal. When the reduction temperature increased from 160 to 600 °C, the copper crystallite size of Cu/ SiO2 and 2.0-Zn-Cu/SiO2 catalysts grew from 3.5 to 4.4 nm and 3.3 to 3.8 nm, respectively, estimated by the Scherrer equation. In general, the growth in copper particle size was retarded with

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X. Wang et al. / Journal of Catalysis 383 (2020) 254–263 Table 1 Characterization data of the key catalysts before and after catalytic testing, and the support.

a b c d e f

Samples

SBET (m2 g1)

Vpore (cm3 g1)a

Dpore (nm)

Cu (wt.%)b

Zn (wt.%)b

X+Cu (%)c

S0Cu (m2 g1)d

SiO2 Cu/SiO2 Zn/SiO2 Cu/SiO2-60he Cu/SiO2-100he 1.0-Zn-Cu/SiO2 1.0-Zn-Cu-60he 1.4-Zn-Cu/SiO2 2.0-Zn-Cu/SiO2 2.0-Zn-Cu-60he 2.0-Zn-Cu-800he 4.0-Zn-Cu/SiO2 4.0-Zn-Cu-60he 2.0-Zn-Cu/SiO2-reff 2.0-Zn-Cu-re-60he

295 439 144 300 340 425 288 411 405 303 330 381 298 377 300

1.40 0.76 0.92 0.63 0.63 0.79 0.61 0.78 0.73 0.71 0.73 0.78 0.67 0.67 0.78

27.4 11.3 27.4 11.7 11.7 13.5 11.7 13.5 13.5 13.5 13.5 13.5 13.5 13.5 13.5

0.0 21.3 0.0 – – 22.0 4.5 21.6 21.6 – – 20.3 – 21.4 –

0.0 0.0 20.5 – – 1.0 – 1.4 2.1 – – 4.2 – 2.0 –

– 47.5 – – – 49.2 – 52.5 58.2 – – 65.5 – – –

– 57.7 – 23.1 13.8 58.4 30.5 59.9 60.7 60.4 52.2 59.8 58.4 58 30.5

Calculated from nitrogen adsorption at p/p0 = 0.98 ICP-OES. Estimated by deconvolution of Cu XAES. Cu metallic surface area calculated by N2O titration. Catalyst after use in the hydrogenation. Reference bimetallic catalyst.

results show that smaller copper particle can be formed with controlled zinc content of 1–2 wt% as compared to the un-promoted catalyst (3.6 vs. 4.7 nm), whereas a further increase in zinc content led to a slightly bigger nanoparticle of 3.9 nm for the 4.0-Zn-Cu/ SiO2. The surface area of metallic copper (S0Cu) are generally viewed as crucial factors affecting the catalytic activity and stability in chemoselective hydrogenations. Nevertheless, these parameters of the copper-based catalysts were found to vary only in the small range of 58–61%, by dissociative N2O chemisorption (Table 1).

3.3. Electronic properties

Fig. 1. (a) In situ XRD patterns of selected catalysts recorded from 25 to 600 °C under reductive ambience (H2, flow rate: 30 mL min1) and (b) the corresponding crystallite sizes estimated by Sherrer equation using the Cu (1 1 1) facet at 43.2° 2h.

the presence of zinc, which is more pronounced at high reduction temperature (>400 °C, Fig. 1b). The average particle size of the reduced catalysts was further characterized by TEM (Fig. 2). The

The chemical state of the metals on the reduced catalysts was measured by XPS (Fig. 3a, c) and XAES (Fig. 3b, d). The Cu 2p3/2 XPS spectrum of Cu/SiO2 centered at ca. 932.4 eV accompanied with a shoulder peak at the high binding energy. The absence of a shakeup satellite peak located approximately 10 eV higher than the binding energy of Cu 2p3/2 suggests that most of the Cu2+ species can be reduced to Cu0 and/or Cu+ [52,53]. Further, a gradual shift of the peak to higher binding energy was observed with increasing Zn content, indicating the strong interactions between the metals due to the charge transfer. Since Cu0 and Cu+ species are indistinguishable from XPS, the relative contribution of these species was estimated by deconvolution of the XAES. The asymmetric and broad Auger kinetic energy peaks were fitted into two symmetrical peaks centered at 915.1 and 912.1 eV which are assigned to Cu+ and Cu0, respectively [54,55]. The results (Table 1) reveal that the surface ratio of Cu+/(Cu0 + Cu+) increased dramatically with an increase of the Zn content, further hinting the electron transfer from copper to zinc. To corroborate the above results, the Zn 2p XPS and XAES spectra were further analyzed. Accordingly, the Zn 2p3/2 XPS of all the Cu-based catalysts showed a major contribution at ca. 1045 eV [56], and a slight downshift in binding energy was evidenced by increasing the zinc content. XAES was applied to discriminate the different zinc species in the catalysts. The deconvolution results show that a significant ratio of zinc was in metallic state for the 1.0-Zn-Cu/SiO2, suggesting the remarkable reduction of ZnO by H2 probably induced by the strong electronic interactions with the copper species. Indeed, the Zn0/ Zn2+ ratio decreased with the increasing Zn/Cu ratio that is particularly pronounced for the 4.0-Zn-Cu/SiO2 with Zn2+ species predominating.

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Fig. 2. TEM images of key catalysts with respective particle size distribution (insets).

Fig. 3. XPS spectra of (a) Cu 2p, and (c) Zn 2p, as well as respective LMM XAES spectra (b, d) of the key catalysts after reduction treatment.

3.4. Evolution of catalyst structure Detailed structures of selected bimetallic catalysts were examined by HRTEM. A detailed look on 2.0-Zn-Cu/SiO2 evidenced that the Cu (1 1 1) facet with an interplanar spacing (d) of 0.208 nm pre-

dominates, and in extreme cases the ZnO ((1 0 0) facet, d = 0.248 nm) and a Cu3Zn alloy ((1 1 1) facet, d = 0.212 nm, (JCPDS #65–6567)) were detected (Fig. 4a, S4). The HAADF-STEM and SEM elemental color mapping results show homogeneous distribution of Cu, Zn, O, and Si, further supporting the high dispersion and good contact between the two metals (Fig. 4c, S5). On the contrary, HRTEM analysis of the 4.0-Zn-Cu/SiO2 reveals the general presence of ZnO as evidenced by the lattice fringe of 0.243 nm corresponding to the ZnO (1 0 1) facet (Fig. 4b, S6). The variation of different copper speciation upon zinc addition was further studied by H2TPD (Figure S7). Two broad desorption peaks are found for all the copper-based catalysts, ranging from 50 to 400 °C. The lowtemperature domain around 100 °C can be ascribed to weakly chemisorbed H2 on the surface copper species, and the desorption at the high temperatures (>300 °C) can be assigned to strong chemisorbed H2 on the bulk phase of copper nanoparticles that are strongly interacting with the promoter and the support, and therefore are more relevant in catalysis [57–59]. When increasing the zinc content, the high-temperature desorption gradually weakens in intensity. The reduced H2 storage ability suggests the presence of difference copper speciation that are likely associated with the formation of Cu-Zn interface as suggested by the strong electronic interactions of the two metals, as well as their intimate contact. This is further supported by in situ FTIR with CO probe, showing clear red shift (ca. 1.6 cm1) of CO on the positively charged copper species (2020–2023 cm1) upon zinc addition (Fig. 4d) [60]. The overall impacts of zinc doping on the copper-based catalysts are summarized in Fig. 4e. At first, the surface Zn/Cu molar ratio increases more linearly with the bulk ratio at a lower doping amount, suggesting that zinc species are finely distributed and bound to the copper analogues. At higher doping level, the increase of surface Zn/Cu ratio slowed down, indicating the poorer contact between the two metal speciation as already revealed by HRTEM analysis. Consequently, a maximal S0Cu was reached on the 2.0Zn-Cu/SiO2, albeit the share of Cu+ kept on increasing with the zinc content. On the basis of the above findings, the evolution of catalyst structure with increasing zinc doping was depicted (Fig. 4f),

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259

Fig. 4. HRTEM images of (a) 2.0-Zn-Cu/SiO2 and (b) 4.0-Zn-Cu/SiO2 catalysts with corresponding electron diffraction (insets); (c) HAADF-STEM elemental color mapping of 2.0-Zn-Cu/SiO2; (d) In situ FT-IR spectra with CO probe over Zn-Cu/SiO2 catalysts; (e) the share of charged copper species (X+Cu), the surface atomic ratio of Zn:Cu, and the surface area of metallic copper, as a function of the bulk Zn:Cu molar ratio; and (f) Catalyst structure evolution with increasing zinc doping.

highlighting the importance of appropriate doping in order to achieve desirable structural and electronic properties for the copper-based systems.

3.5. Comparison of catalytic performance in DMO hydrogenation The catalytic performance of the developed copper-based systems in vapor-phase DMO hydrogenation was evaluated in a fixed-bed reactor. The reaction comprises multi-hydrogenation steps, leading to different side-products including MG and ethanol by hydrogenation, and 1,2-propanediol and 1,2-butanediol by dehydrogenation [6,38]. Preliminary optimization of reaction conditions was conducted on the 2.0-Zn-Cu/SiO2 (Fig. 5). The influence of total pressure was first studied under conditions of H2/DMO = 30, WLHSV = 1.0 h1, and T = 200 °C. Under such conditions DMO conversion increases with elevated pressure up to 1.5 MPa and then levels off, with MG as the main product. To enhance the selectivity to EG, the influence of H2/DMO ratio was further studied. In line with the literature results [48], both the DMO conversion and the EG selectivity increase with higher H2/DMO ratio until full DMO conversion was reached at H2/DMO = 130, with EG selectivity > 96.7%. Lastly, the influence of WLHSV was examined at H2/DMO = 130, T = 200 °C, and P = 1.5 MPa, and the results show an optimal catalytic performance at WLHSV ~ 1.0. Therefore, an optimized reaction condition of H2/DMO = 130, WLHSV = 1.0 h1 and P = 1.5 MPa, was fixed for further studies. A direct comparison on the catalytic performance of copperbased catalysts was conducted on the selected samples by using temperature ramping experiment from 160 to 210 °C (Fig. 6a). Typical light off curves are found for all the samples, and the temperature was upshifted upon increasing zinc content, hinting the weakened hydrogenation performance of the doped catalysts. A

full comparison on the hydrogenation performance of all the copper-based systems was then made at the bed-temperature of 180 and 200 °C (Fig. 6d, e). At the low temperature the DMO conversion clearly decreased with the zinc content with significant formation of MG over all the catalysts, suggesting the insufficient hydrogenation of MG. However, raising the temperature to 200 °C led to almost full DMO conversion for all the catalysts. Interestingly, the EG selectivity peaked for the 2.0-Zn-Cu/SiO2. While a higher zinc content favored the MG formation, other sideproducts increased substantially with a lower Zn amount. Therefore, these results demonstrate that delicate tuning of the relative Zn/Cu ratio is indispensable in order to avoid under- or overhydrogenation. To verify this point, the hydrogenations of MG and EG were further studied (Fig. 6b, c). Like in the temperature ramping experiments for DMO hydrogenation, similar upshift of the curves with increasing zinc content was evidenced. In order to qualitatively describe the catalyst hydrogenation ability, three parameters were defined, i.e., T50,DMO (the bed temperature at 50% DMO conversion), T50,MG (the bed temperature at 50% MG conversion), and T10,EG (the bed temperature at 10% EG conversion), and presented in Fig. 6f. It clearly shows that the Cu/SiO2 has the lowest T10,EG and thus it can easily promotes further EG hydrogenation, accounting for the pronounced selectivity of side-products. On the contrary, the 4.0-Zn-Cu/SiO2 has the highest T50,DMO and T50,MG and thus the weakest DMO hydrogenation ability. The triangle associated with the 2.0-Zn-Cu/SiO2 locates in the center of the radar plot, indicating a more balanced hydrogenation ability for DMO/MG and EG.

3.6. Structure-performance relationships To quantitatively describe the catalyst hydrogenation ability, the surface Cu0/Cu+ molar ratio as a potential catalytic descriptor

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Fig. 5. Optimization of DMO hydrogenation performance over 2.0-Zn-Cu/SiO2 catalyst. Catalytic performance as a function of (a) total pressure at 200 °C, 1.0 h1, and H2/ DMO = 30, (b) H2/DMO ratio at 200 °C, 1.5 MPa, and WLHSV = 1.0 h1, and (c) WLHSV at 200 °C, 1.5 MPa, and H2/DMO = 130.

Fig. 6. The conversion levels of (a) DMO, (b) MG, and (c) EG in the respective hydrogenation performance over selected catalysts as a function of the reaction temperature, and the product distribution of DMO hydrogenation at (d) 180 °C, and (e) 200 °C. (f) Radar plot of T50,DMO, T50,MG, and T10,EG, of selected catalysts. The triangles closer to the center are more favorable for the selective formation of EG. Reaction conditions: 1.5 MPa, H2/(DMO, MG or EG) = 130, and WLHSV = 1.0 h1.

was considered, as balanced Cu0/Cu+ sites were found to be crucial in the selective DMO hydrogenation to ethanol [25]. Accordingly, correlations of the catalytic performance of different copperbased catalysts with the Cu0/Cu+ ratio, derived from XAES, were made (Fig. 7a, b). At 180 °C, both the DMO conversion and EG selectivity shows positive dependence on the Cu0/Cu+ ratio while MG selectivity shows an opposite trend. This suggests that at the low temperature DMO hydrogenation might be the rate-limiting step. Since metallic copper is likely the active center for the H2 activation, enhanced activity can be expected by increasing the number of the Cu0 sites. On the other hand, increasing the reaction temperature can accelerate the kinetics thus leading to near full DMO conversion over all the catalysts. In this case, the dependence of EG selectivity on the Cu0/Cu+ ratio presents a volcano-like curve, with

the maximal selectivity reached at Cu0/Cu+ = 0.72 (corresponding to the 2.0-Zn-Cu/SiO2). The formation of MG and other sideproducts built up at lower or higher values, respectively, due to under- and over-hydrogenations. These results suggest that the Cu0/Cu+ ratio of the copper-based catalysts can be viewed as a good descriptor for the activity and selectivity in hydrogenation, the latter being more temperature-dependent. To corroborate the above findings, H2-TPR was performed to qualitatively depict the reducibility of the copper-based catalysts (Fig. 7c). A major reduction at 230 °C accompanied with a shoulder peak at higher temperature was observed for all the catalysts, which can be ascribed to the reduction of copper silicate to Cu+ or the highly dispersed CuO crystallites to Cu0 [19,61,62], and the reduction of the bulk CuO crystallites to metallic Cu, respectively. A clear upshift of both

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Fig. 7. Correlations of DMO conversion and product selectivity with the surface atomic Cu0/Cu+ ratio over Zn-doped Cu/SiO2 catalyst, (a) 180 °C and (b) 200 °C. (c) H2-TPR profiles of the key catalysts after calcination.

reduction peaks with increasing zinc content was evidenced, demonstrating the feasibility of gradually tuning the catalyst reducibility through controlled zinc doping. Previous studies on the Cu/SiO2 catalyst prepared by ammoniaevaporation method suggest that the synergy of Cu0-Cu+ is crucial for achieving high performance in DMO hydrogenation to EG [25], which is further supported by a detailed kinetics study [23]. Based on the optimal model proposed, the dissociative adsorption of ester and molecular H2 occurs on the respective Cu+ and Cu0 sites, and the former is the rate-limiting step. Accordingly, enhanced reaction rate would be expected by increasing the number of the Cu+ sites, but this is not the case in this study. Also, our results differ significantly from a recent study on the CuO-ZnO/SiO2 catalyst prepared by a coprecipitation method [20]. Albeit increased Cu+ ratio upon zinc addition has been confirmed in both catalyst systems, the dependences of DMO conversion on the Cu0/Cu+ ratio are totally divergent. The contrasting catalytic behavior of monoand bimetallic systems, as well as the Cu-Zn/SiO2 catalysts of different preparation methods, highlights the crucial role of the dopant in tuning the catalytic performance and the necessity in examining the catalyst structural geometry in the molecular level in order to differentiate the catalysts with similar compositions. 3.7. Relevance of Cu-Zn interface in stability performance The long-term stability tests were performed at 200 °C on the Cu/SiO2 and 2.0-Zn-Cu/SiO2 catalysts (Fig. 8a). While both systems can afford high initial activity and EG selectivity, fast deactivation occurred at 80 h on stream on the un-promoted catalyst with an activity loss of ca. 60% after 150 h. In stark contrast, the 2.0-ZnCu/SiO2 showed no apparent catalyst deactivation within 800 h on-stream test, and the conversion and EG selectivity stayed at >99% and >95%, respectively, demonstrating the great potential for industrial applications. To understand the promotional effect of zinc doping on the stability of the copper-based catalysts, the used catalysts were thoroughly characterized by combined techniques. First, structural properties were analyzed by N2 sorption, revealing ca. 23% and 19% reduction in the specific surface area for the Cu/SiO2 and 2.0-Zn-Cu/SiO2 catalysts before and after the catalytic tests (Table 1). Then the surface area of copper were estimated on the basis of N2O chemisorption. Albeit drops in these parameters are evidenced after use for both catalysts, the scenario was much milder for the zinc-promoted catalyst. For instance, the loss of surface area of metallic copper is 76% for Cu/SiO2 and only 14% for 2.0Zn-Cu/SiO2. This finding is further supported by TEM observations (Fig. 8c). Severe particle agglomeration was revealed on the used Cu/SiO2 (from 4.7 to 9.3 nm) after 150 h test while the mean par-

ticle size only increased from 3.3 to 5.5 nm for the 2.0-Zn-Cu/SiO2 even after much longer on-stream test. Therefore, the stabilization of copper nanoparticle under hydrogenation conditions induced by zinc doping is one of the major reasons accounting for the excellent stability performance. Still, the intrinsic role of zinc in stabilizing copper nanoparticles, particularly how it modulates the copper catalyst, is not clear. On the basis of the comprehensive characterization results, we speculate that the intimate contact between Cu and Zn might lead to the formation of bimetallic interface (but not Cu-Zn alloy as confirmed by HRTEM) due to the strong electronic interactions of the metals as revealed by XPS and XAES. To further shed light on this point, an additional bimetallic catalyst with poorer metal contact, referred as 2.0-Zn-Cu/SiO2-ref, was prepared by following similar method for the 2.0-Zn-Cu/SiO2 but with fast drying after zinc loading. This catalyst features similar particle size distribution (Dp = 3.5 nm) with the 2.0-Zn-Cu/SiO2 benchmark catalyst but a substantial number of ZnO particles are quite visible from HRTEM (Fig. 8d) that is also very different from the 4.0-Zn-Cu/SiO2. Therefore, it can serve as an ideal reference to access the interface impact on the stability performance. To this end, accelerated deactivation experiments at much harsher conditions were designed on purpose to shorten the evaluating time while keeping the conversion level at ca.70% to ensure that all the active sites are utilized (Fig. 8b). Results on the key samples show that the stability performance follows the order of Cu/SiO2 < 1.0-Zn-Cu/SiO2 < 2.0-Zn-Cu/ SiO2-ref < 4.0Zn-Cu/SiO2 ~2.0 Zn-Cu/SiO2. This trend is in excellent agreement with the variation of S0Cu (Table 1), with only minor changes observed for the last two catalysts. Clearly, the standard catalyst series with intimate Cu-Zn contact evidences the optimal zinc doping in order to achieve the superb stability. Furthermore, the reference catalyst deactivated much faster than the benchmark with nearly the same loading and mean particle size of the copper species. TEM analysis of the catalyst after use reveals significant agglomeration of the nanoparticles from 3.5 to 7.3 nm after 60 h testing (Figure S8), in line with the substantially reduced metal dispersion. Therefore, the above observations strongly point to the Cu-Zn interface in stabilizing the copper nanoparticle and in determining the stability performance in the hydrogenation. Considering the structural and compositional complexity of the catalytic system, developing bimetallic Zn-Cu model systems in conjunction with theoretically investigation would definitely shed more insight on this aspect, which is out of the scope of this study. 4. Conclusions Silica-supported copper-zinc bimetallic catalysts with intimate contact between the metals have been prepared through an

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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This research was supported by the National Natural Science Foundation of China (No. 21803056), and Zhejiang Provincial Natural Science Foundation of China (No. LQ18B030003). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcat.2020.01.018. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] Fig. 8. (a) Stability performance of the Cu/SiO2 (triangle) and 2.0-Zn-Cu/SiO2 (circle) in DMO hydrogenation. Reaction conditions: 200 °C, 1.5 MPa, WLHSV = 1.0 h1, and H2/DMO = 130. (b) Stability performance of the key catalyst series and the reference sample 2.0-Zn-Cu/SiO2-re under accelerated deactivation conditions in DMO hydrogenation. Reaction conditions: 280 °C, 1.5 MPa, WLHSV = 5 h1, and H2/DMO = 30. (c) TEM images of the catalysts after stability tests in (a) accompanied with corresponding particle size distribution (insets). (d) TEM and HRTEM images of 2.0-Zn-Cu/SiO2-re catalyst after stability test in (b) accompanied with corresponding particle size distribution (inset, left).

[21] [22] [23] [24] [25] [26]

ammonia evaporation-impregnation method. Finely controlling the doping level allows systematically tuning the geometrical and electronic properties of the copper nanoparticles. In particular these strong electronic interactions due to electron transfer from copper to ZnO in combination with the intimate metal contact facilitate the formation of bimetallic interface. A proper balance of these properties leads to excellent performance in the vaporphase chemoselective hydrogenation of DMO to EG with high selectivity and unprecedented stability. Our results further highlight that i) the Cu0/Cu+ ratio can be regarded as a good activity and selectivity descriptor in the hydrogenation, and ii) the critical role of Cu-Zn interface for sintering-resistant hydrogenation catalyst. Overall, the strategy demonstrated in this study provides a new prospective to stabilizing metal nanoparticles, and to enhancing the hydrogenation performance and beyond.

[27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]

Y. Liu, J. Ding, J. Yang, J. Bi, K. Liu, J. Chen, Catal. Commun. 98 (2017) 43–46. A. Yin, X. Guo, W. Dai, K. Fan, Chem. Commun. 46 (2010) 4348–4350. H. Song, R. Jin, M. Kang, J. Chen, Chin. J. Catal. 34 (2013) 1035–1050. Q. Yang, C. Zhang, D. Zhang, H. Zhou, Ind. Eng. Chem. Res. 57 (2018) 7600– 7612. Y.F. Zhu, X. Kong, H.Y. Zheng, Y.L. Zhu, Mol. Catal. 458 (2018) 73–82. C. Wen, Y. Cui, A. Yin, K. Fan, W.-L. Dai, ChemCatChem 5 (2013) 138–141. C. Wen, Y. Cui, W.L. Dai, S. Xie, K. Fan, Chem. Commun. 49 (2013) 5195–5197. C. Wen, Y. Cui, X. Chen, B. Zong, W.-L. Dai, Appl. Catal. B Environ. 162 (2015) 483–493. M.M. Li, L. Ye, J. Zheng, H. Fang, A. Kroner, Y. Yuan, S.C. Tsang, Chem. Commun. 52 (2016) 2569–2572. J. Zhou, X. Duan, L. Ye, J. Zheng, M.M.-J. Li, S.C.E. Tsang, Y. Yuan, Appl. Catal. A Gen. 505 (2015) 344–353. A. Yin, C. Wen, W.-L. Dai, K. Fan, Appl. Catal. B Environ. 108–109 (2011) 90–99. J. Zheng, H. Lin, X. Zheng, X. Duan, Y. Yuan, Catal. Commun. 40 (2013) 129–133. Y. Zhao, B. Shan, Y. Wang, J. Zhou, S. Wang, X. Ma, Ind. Eng. Chem. Res. 57 (2018) 4526–4534. J. Wu, G. Gao, P. Sun, X. Long, F. Li, ACS Catal. 7 (2017) 7890–7901. D. Wang, G. Yang, Q. Ma, M. Wu, Y. Tan, Y. Yoneyama, N. Tsubaki, ACS Catal. 2 (2012) 1958–1966. K. Chen, H. Fang, S. Wu, X. Liu, J. Zheng, S. Zhou, X. Duan, Y. Zhuang, S. Chi Edman Tsang, Y. Yuan, Appl. Catal. B Environ. 251 (2019) 119–129. S.D. Jones, L.M. Neal, H.E. Hagelin-Weaver, Appl. Catal. B Environ. 84 (2008) 631–642. D. Yao, Y. Wang, Y. Li, Y. Zhao, J. Lv, X. Ma, ACS Catal. 8 (2018) 1218–1226. Y. Zhao, S. Li, Y. Wang, B. Shan, J. Zhang, S. Wang, X. Ma, Chem. Eng. J. 313 (2017) 759–768. W. Qi, Q. Ling, D. Ding, C. Yazhong, S. Chengwu, C. Peng, W. Ye, Z. Qinghong, L. Rong, S. Hao, Catal. Commun. 108 (2018) 68–72. B. Zhang, S. Hui, S. Zhang, Y. Ji, W. Li, D. Fang, J. Nat. Gas Chem. 21 (2012) 563– 570. Y. Wang, W. Yang, D. Yao, S. Wang, Y. Xu, Y. Zhao, X. Ma, Catal. Today (2019), https://doi.org/10.1016/j.cattod.2019.06.031. S. Li, Y. Wang, J. Zhang, S. Wang, Y. Xu, Y. Zhao, X. Ma, Ind. Eng. Chem. Res. 54 (2015) 1243–1250. F. Li, C.-S. Lu, X.-N. Li, Chin. Chem. Lett. 25 (2014) 1461–1465. J. Gong, H. Yue, Y. Zhao, S. Zhao, L. Zhao, J. Lv, S. Wang, X. Ma, J. Am. Chem. Soc. 134 (2012) 13922–13925. M. Ouyang, J. Wang, B. Peng, Y. Zhao, S. Wang, X. Ma, Appl. Surf. Sci. 466 (2019) 592–600. Y. Sun, F. Meng, Q. Ge, J. Sun, ChemistryOpen 7 (2018) 969–976. K. Chen, J. Yu, B. Liu, C. Si, H. Ban, W. Cai, C. Li, Z. Li, K. Fujimoto, J. Catal. 372 (2019) 163–173. H. Yue, Y. Zhao, S. Zhao, B. Wang, X. Ma, J. Gong, Nat. Commun. 4 (2013) 2339. J. Ding, T. Popa, J. Tang, K.A.M. Gasem, M. Fan, Q. Zhong, Appl. Catal. B Environ. 209 (2017) 530–542. J. Wu, G. Gao, J. Li, P. Sun, X. Long, F. Li, Appl. Catal. B Environ. 203 (2017) 227– 236. Z.X. Li, Z.G. Zhao, X.G. Zhao, W.D. Xiao, Petrochem. Technol. 33 (2004) 744– 746. A. Yin, J. Qu, X. Guo, W.-L. Dai, K. Fan, Appl. Catal. A Gen. 400 (2011) 39–47. Z. He, H. Lin, P. He, Y. Yuan, J. Catal. 277 (2011) 54–63. Y. Chen, L. Han, J. Zhu, P. Chen, S. Fan, G. Zhao, Y. Liu, Y. Lu, Catal. Commun. 96 (2017) 58–62. X. Kong, Z. Chen, Y. Wu, R. Wang, J. Chen, L. Ding, RSC Adv. 7 (2017) 49548– 49561. C. Zhang, D. Wang, M. Zhu, F. Yu, B. Dai, Catal. Commun. 102 (2017) 31–34. A. Yin, C. Wen, X. Guo, W.-L. Dai, K. Fan, J. Catal. 280 (2011) 77–88. X. Zheng, H. Lin, J. Zheng, X. Duan, Y. Yuan, ACS Catal. 3 (2013) 2738–2749.

X. Wang et al. / Journal of Catalysis 383 (2020) 254–263 [40] P. Ai, M. Tan, P. Reubroycharoen, Y. Wang, X. Feng, G. Liu, G. Yang, N. Tsubaki, Catal. Sci. Technol. 8 (2018) 6441–6451. [41] J.L. Santos, T.R. Reina, S. Ivanova, M.A. Centeno, J.A. Odriozola, Appl. Catal. B Environ. 201 (2017) 310–317. [42] R.M. Palomino, P.J. Ramirez, Z. Liu, R. Hamlyn, I. Waluyo, M. Mahapatra, I. Orozco, A. Hunt, J.P. Simonovis, S.D. Senanayake, J.A. Rodriguez, J. Phys. Chem., B 122 (2018) 794–800. [43] S. Kattel, P.J. Ramírez, J.G. Chen, J.A. Rodriguez, P. Liu, Science 355 (2017) 1296–1299. [44] L. Zheng, X. Li, W. Du, D. Shi, W. Ning, X. Lu, Z. Hou, Appl. Catal. B Environ. 203 (2017) 146–153. [45] F. Liao, Y. Huang, J. Ge, W. Zheng, K. Tedsree, P. Collier, X. Hong, S.C. Tsang, Angew Chem. Int. Ed. 50 (2011) 2162–2165. [46] O. Martin, C. Mondelli, A. Cervellino, D. Ferri, D. Curulla-Ferré, J. Pérez-Ramírez, Angew Chem. Int. Ed. 55 (2016) 11031–11036. [47] D. Grandjean, V. Pelipenko, E.D. Batyrev, J.C. van den Heuvel, A.A. Khassin, T.M. Yurieva, B. Weckhuysen, J. Phys. Chem. C 115 (2011) 20175–20191. [48] G. Cui, X. Meng, X. Zhang, W. Wang, S. Xu, Y. Ye, K. Tang, W. Wang, J. Zhu, M. Wei, D.G. Evans, X. Duan, Appl. Catal. B Environ. 248 (2019) 394–404. [49] Y. Zhao, Z. Guo, H. Zhang, B. Peng, Y. Xu, Y. Wang, J. Zhang, Y. Xu, S. Wang, X. Ma, J. Catal. 357 (2018) 223–237.

263

[50] L. Chen, P. Guo, M. Qiao, S. Yan, H. Li, W. Shen, H. Xu, K. Fan, J. Catal. 257 (2008) 172–180. [51] Z. Yuan, L. Wang, J. Wang, S. Xia, P. Chen, Z. Hou, X. Zheng, Appl. Catal. B Environ. 101 (2011) 431–440. [52] M. Abbas, Z. Chen, J.J. Chen, Mater. Chem. A 6 (2018) 19133–19142. [53] D. Wang, C. Zhang, M. Zhu, F. Yu, B. Dai, ChemistrySelect 2 (2017) 4823–4829. [54] Y. Cui, B. Wang, C. Wen, X. Chen, W.-L. Dai, ChemCatChem 8 (2016) 527–531. [55] Y. Wang, Y. Shen, Y. Zhao, J. Lv, S. Wang, X. Ma, ACS Catal. 5 (2015) 6200–6208. [56] l. Zhen, Q. Wang, J. Liu, T. Yu, W. Wang, Appl. Catal. A Gen. 239 (2003) 87–94. [57] F. Arena, G. Italiano, K. Barbera, S. Bordiga, G. Bonura, L. Spadaro, F. Frusteri, Appl. Catal. A Gen. 350 (2008) 16–23. [58] S. Xia, Z. Yuan, L. Wang, P. Chen, Z. Hou, Appl. Catal. A Gen. 403 (2011) 173– 182. [59] X. Guo, D. Mao, G. Lu, S. Wang, G. Wu, J. Mol. Catal. A: Chem. 345 (2011) 60–68. [60] S. Yin, L. Zhu, X. Wang, Y. Liu, S. Wang, Chin. J. Chem. Eng. 27 (2019) 386–390. [61] S. Zhao, H. Yue, Y. Zhao, B. Wang, Y. Geng, J. Lv, S. Wang, J. Gong, X. Ma, J. Catal. 297 (2013) 142–150. [62] X. Kong, C. Ma, J. Zhang, J. Sun, J. Chen, K. Liu, Appl. Catal. A Gen. 509 (2016) 153–160.